Board Thread:Pokémon Help/@comment-32069757-20170703022210/@comment-28826236-20170703024512

You can control it.

"It evolves into one of three different Pokémon starting at level 20: Hitmonlee if its Attack is higher than its Defense. Hitmonchan if its Defense is higher than its Attack. Hitmontop if its Attack and Defense are the same."

~Bulbapedia

I think this concerns the Attack and Defense EVs. If its Attack EV is higher than its Defense EV, it evolves into Hitmonlee. If the EV's are the other way around, it evolves into Hitmonchan. And if they are at the exact same level....it evolves into Hitmontop.